An image sensor may be a semiconductor device that may convert an optical image into an electric signal. An image sensor may be classified as at least one of a charge coupled device (CCD) image sensor and a complementary metal oxide silicon (CMOS) image sensor (CIS).
A CIS may include a photodiode and a MOS transistor formed in a unit pixel, and may obtain an image by sequentially detecting electrical signals of unit pixels in a switching manner. In a CIS, a photodiode region may convert a light signal to an electrical signal, and a transistor may process the electrical signal. A photodiode region and a transistor may be horizontally arranged on and/or over a semiconductor substrate. In a horizontal type CIS according to the related art, a photodiode and a transistor may be horizontally formed adjacent to each other on and/or over a substrate. Therefore, an additional region for forming a photodiode may be required.